1. Field of the Invention
The present invention relates to an air-tightly sealed container provided with a light transmissive window unit for housing a photosemiconductor element therein, and a photosemiconductor module using this air-tightly sealed container for photosemiconductors.
2. Description of the Prior Art
In the optical communication, the air-tightness of a photosemiconductor module is regarded as important for the purpose of securing a high reliability. The reasons resides in that, in a high-temperature and high humidity condition, the electrodes of a photosemiconductor element is somewhat deteriorated with the water which enters the interior of the container and dews to cause the optical characteristics to be deteriorated. This makes it impossible to ensure the lifetime of not less than 10 years of the photosemiconductor element.
In a photosemiconductor module, a photosemiconductor element in a container and an optical fiber on the outer side of the container have to be connected together. In order to optically connect together the photosemiconductor element in the container and optical fiber on the outer side thereof with the air-tightness of the container secured, a light transmissive window structure is employed in the air-tightly sealed container for photosemiconductors.
As an example of such a light transmissive window structure, Japanese Patent Laid-Open No. 6-151629 discloses a window structure using a borosilicate glass plate as a window member, which is first seal-bonded to a ceramic or metallic carrier by using low melting point glass, the resultant complex body being then fixed to a container for photosemiconductors by brazing. Japanese Patent Laid-Open No. 8-148594 discloses a light transmissive window structure formed by brazing metallized sapphire to an inclined surface of a cylindrical fixed portion by using a Au--Sn alloy.
A glass plate used as a window member of a light transmissive window is generally formed to a substantially right hexagonal shape as a shape close to a circular shape, and coated at the whole surface with a nonreflective film. When the window member is constituted of an expensive material, such as sapphire, it is formed into a circular shape in a usual case by using a twist drill polishing machine so as to increase the yield of window member chips. However, since the borosilicate glass is inexpensive, the reduction of the manufacturing cost is principally aimed, i.e., the borosilicate glass is formed to a substantially right hexagonal window member by using an inexpensive linear machining process by dicing. In a machining process using only a linear action, small triangle-shaped cutting margins occur excessively. Since the borosilicate glass is cut to a substantially right hexagonal shape, the highest yield can be attained, though a linear machining method is used.
In such an air-tightly sealed container for photosemiconductors, a combination of different kinds of materials, such as a ceramic material, Cu--W alloy and Fe--Ni--Co alloy is used in view of the necessity of improving the heat-radiation characteristics and high-frequency characteristics. Consequently, thermal stress strain, which is ascribed to differences between the thermal expansion coefficients of different materials, has come to occur greatly during a brazing operation in the light transmissive window unit.
As disclosed in Japanese Patent Laid-Open No. 6-151629, i.e., in the method of seal-bonding a borosilicate glass plate constituting a window member to a ceramic or metallic carrier by low melting point glass, and then brazing the resultant composite body to a plate material of a large thickness, such as a container for photosemiconductors, the breakage of the glass plate occurs during a brazing operation in some cases where the thickness of the glass plate is small. This is caused by the fragility of the borosilicate glass which is higher than that of sapphire. The percentage of occurrence of imperfect products using borosilicate glass plates is as high as around 30%.
When a borosilicate glass plate is thick, the breakage thereof did not occur in a -65.degree. C..about.25.degree. C..about.150.degree. C. heat cycle test based on MIL-STD but leakage based on MIL-STD occurred in around 20% products. This leakage occurs in a brazed portion between a carrier and a container for photosemiconductors. Namely, when the thickness of a glass plate increases, the glass plate becomes difficult to be broken owing to an increase in the strength but the glass plate becomes difficult to be deformed, so that a large stress is concentrated on the brazed portion to cause the brazing material to be broken in the heat cycle test.
In the case of a window member formed of a borosilicate glass plate, these two failure modes concerning the thickness thereof are thus superposed on each other, and, whatever thickness the glass plate is composed of, the glass plate falls into either of the two failure modes. Therefore, it was difficult to air-tightly seal a container in a satisfactory manner.
In a window structure using sapphire which is disclosed in Japanese Patent Laid-Open No. 8-148594, inconveniences concerning the above-mentioned two failure modes do not occur. The reasons reside in that the sapphire is high in strength, not fragile, and high in adhesion with respect to a metallized part. Accordingly, the sapphire enables a window member to be formed to a satisfactorily small thickness, and the occurrence of breakage of a glass or brazing material, which is encountered in a case where a borosilicate glass is used, to be prevented.
The sapphire is very expensive as known from the price of the same as a gem. As is understood from a recent internet unit, the cost reduction is essential to the popularization of optical communication, so that using an expensive sapphire as a window member is indeed a problem.